In the mustached bat (Pteronotus parnellii), cortical auditory
neurons mediate, via corticofugal projection, a highly focused
positive feedback to subcortical neurons "matched"
in tuning to a particular acoustic parameter in the frequency
or time domain, and a widespread lateral inhibition to "unmatched"
subcortical neurons. This cortical feedback results in the adjustment
and improvement of subcortical signal processing (i.e., the
adjustment and improvement of the cortical neurons' own input).
This function, named "egocentric selection," enhances
the neural representation of frequently occurring signals in
the central auditory system (Yan & Suga 1996; Zhang, Suga &
Yan 1997). In the big brown bat (Eptesicus fuscus), egocentric
selection shifts the best frequencies of collicular neurons
not only toward the best frequency of electrically stimulated
cortical neurons but also towards the frequency of a repetitively
delivered acoustic stimulus (tone burst), resulting in local
reorganization of the frequency map in the subcortical nuclei
(Yan & Suga 1998), and also the auditory cortex (Chowdhury &
Suga 1998). It also evokes such reorganization according to
auditory experience based on associative learning (Gao and Suga,
1998).

The effect of egocentric selection is apparently different
between the two species of bats studied and, perhaps, between
different portions of a frequency map of the central auditory
system of the mustached bat, reflecting the shape and sharpness
of frequency-tuning curves. Therefore, I may propose the hypothesis
that egocentric selection is a basic neural mechanism shared
by many mammalian species, but its effects on subcortical neurons,
and accordingly, cortical neurons, can be somewhat different
from species to species, depending on species-specific needs
for auditory signal processing.

Gao and Suga (1998) proposed the following model, which is
somewhat different from Weinberger's model (1998) based on LeDoux
and Muller's model (1997). A train of acoustic stimuli (AS)
and an electric leg-stimulation (ES) excite the auditory and
somatosensory cortices, respectively. These sensory cortices
send signals to the amygdala through the association cortex.
AS evokes changes in the auditory cortex, which are highly specific
to acoustic stimuli and are based on egocentric selection working
together with the corticofugal (descending) system. (The corticofugal
system forms a feedback loop with the ascending system.) When
associative learning takes place in the amygdala, that is, when
an animal is conditioned by AS followed by ES, the cholinergic
basal forebrain is excited by AS + ES through the amygdala and
increases the acetylcholine level in the cortex. As a result,
the AS-related changes in the auditory cortex are augmented
in magnitude and duration. In other words, the processing of
acoustic stimuli is adjusted and improved according to the behavioral
relevance of the stimuli.

I will try to simplify my story to be understandable to non-auditory
physiologists. (Work supported by NIDCD DC 00175).